GH-110109: Test pure functionality of `pathlib.Path` user subclasses (#112242)

Add `PurePathTest` as a superclass of `PathTest`, and therefore also
`PathSubclassTest`. This adds coverage of pure functionality in user
subclasses of `pathlib.Path`.

Remove `PosixPathAsPureTest` and `WindowsPathAsPureTest`, as they
now duplicate `PosixPathTest` and `WindowsPathTest`.

This makes the MROs of test unit classes match the MROs of pathlib
classes.